It is well known that the angular resolution of a conventional lens is proportional to the wavelength of the light collected, and is inversely proportional to the diameter of the lens. Assuming the wavelength of the light is 1.0 micrometer, and that an angular resolution of 10 nanoradians is desired, the lens would need to have a diameter of 100 meters, and would hence be impractically large to fabricate and/or transport or move by mechanical means.
The problem is similar in radar systems. The resolution provided by typical radar antennas is on the order of 0.1 radians but the resolution desired is on the order of 0.1 milliradians. However, the needed improvement (by a factor of 1000) can be achieved in radar systems by using various doppler beam sharpening techniques, or by the use of the synthetic aperture method. Synthetic Aperture Radar (SAR) synthesizes a large antenna by moving a single small antenna along a line. At many locations along this line, the small antenna transmits signals to the target and collects the reflected signals. SAR techniques for improving the angular resolution provided by a single antenna by many orders of magnitude are generally discussed in Brown, Synthetic Aperture Radar, IEEE Transactions on Aerospace and Elec. Systems, Vol. 3, No. 2, Mar., 1967 and in Brown, An Introduction to Synthetic Aperture Radar, IEEE Spectrum, Sept., 1967. Optical systems which directly parallel SAR systems have also been developed. Examples of optical systems employing SAR techniques are disclosed in U.S. Pat. No. 4,011,445 entitled "Optical Array Active Imaging Technique" and in U.S. Pat. No. 3,895,381 entitled "Synthetic Aperture Imaging Systems."
In conventional optical systems, improved resolution is normally pursued by carefully polishing the lens surface and increasing the focal length. If surface distortions are not to reduce the resolution of the lens even more from the inherent limits imposed by its limited diameter, then the surface must be accurately polished to within about one-tenth the wavelength of the light used. In a distributed aperture optical system, in which light from the target is captured by a two-dimensional array of smaller light receivers or reflectors, each forming a small portion of the area of an overall light receiving surface, an array of pistons may be employed to independently adjust the individual surfaces. U.S. Pat. No. 4,639,586 entitled "Optically Phased Laser Transmitter" describes a variation of this technique in which an array consisting of a small number of laser transmitters is optomechanically phased.
In prior distributed aperture optical systems, the need to position the optical surface with a precision of one-tenth the wavelength of the light, while still adjusting that position over a dynamic range of perhaps several centimeters in order to direct and focus the system, imposes an extremely stringent requirement. For a 1.0 micrometer wavelength, this corresponds to a dynamic range of approximately 10.sup.6.
An integral part of the "figure control" function of a distributed aperture optical imaging system is the calculation, from measurement data, of the adjustments needed to place the array of individual light receivers in coherence. This calculation needs to be performed continuously, and at a rate at least twice as high as the highest temporal frequency of the distortion inputs that can cause the system to fall out of alignment. Further, these corrections must be made at a spatial density sufficiently greater than the highest density of the spatial distortion functions present in the collection aperture. In prior art systems, both the measurement of the figure error and the processing required to generate the update information is computationally intensive and significantly limits the rate at which the update information can be generated. This in turn limits the quality of the obtained figure and affects the amount and types of disturbances that the figure control component of the system can handle. Further, the optical-mechanical actuators required to implement the figure control commands are highly complex, expensive, large and consume significant amounts of power to operate.